Recent developments in semiconductor integrated-circuit technology have been remarkable with the miniaturization of the constituent semiconductor elements and trends toward increased integrated-circuit density. Up to the present, so-called optical lithography steppers have conventionally been used for performing lithographic exposure (projection-transfer) of integrated-circuit patterns onto semiconductor wafers. Unfortunately, current optical lithography techniques are (or soon will be) unable to provide the image resolution necessary to satisfy anticipated demands for ever decreasing miniaturization of semiconductor elements and increases in integrated-circuit density. Consequently, effort has been expended to develop microlithographic equipment employing a charged particle beam (e.g., electron beam or ion beam) or an X-ray beam rather than a light (ultraviolet) beam. That is, a charged particle beam or X-ray beam is used to project a pattern, defined by a mask or reticle (both terms are used interchangeably herein), with demagnification, onto a sensitive substrate, such as a semiconductor wafer coated with a suitable resist.
One type of mask used in conventional charged-particle-beam (CPB) projection-transfer systems is a stencil mask. Referring to FIG. 3, a stencil mask typically comprises a silicon membrane 6 having through-holes 6a formed therein. The silicon membrane 6 is formed from a silicon substrate 5. The silicon substrate 5 acts as support for the membrane. The through-holes 6a, together with the remaining mask membrane, define a pattern. Stencil masks are frequently used in ion-beam lithography and cell-projection electron-beam lithography apparatus.
Referring to FIG. 2, full-size X-ray lithography masks typically comprise a patterned material 3 (typically tantalum) formed on a silicon nitride membrane 4. The silicon nitride membrane 4 is formed on and supported by a silicon substrate 2.
As used herein, a "mask workpiece" is a mask in the process of manufacture. Mask manufacture typically involves one or more anisotropic etching steps that facilitate the formation of the pattern defined by the mask.
During manufacture of masks such as shown in FIGS. 2-3, process temperatures typically increase to levels that result in damage to the mask membrane or in inaccurate mask patterns. If the mask workpiece is not maintained below a specific temperature level during manufacture, certain steps may not be executed properly. For example, excessive temperature may cause an anisotropic etch process to become isotropic, resulting in undesirable angled pattern features that reduce the mask's resolution. Thus, the mask workpiece should be maintained below a specific temperature during all steps of the mask-manufacturing process (the specific temperature is determined by the particular etching process and etching equipment used to make the mask). Maintaining the temperature of the mask workpiece below such a level yields stable etch results and allows performance of anisotropic etching. Accordingly, the resolution of the resulting mask pattern is increased if the temperature of the mask workpiece is controlled as the mask is formed (i.e., during the etch processes).
Conventional temperature control for dry-etch processes involves control of the temperature of the mask membrane as it is etched (i.e., membrane 4 in FIG. 2 and membrane 6 in FIG. 3). A stream of coolant gas (at a pressure of several tens of mTorr) is flowed directly on the lower surface of the membrane (i.e., membrane surface 4' in FIG. 2 and membrane surface 6' in FIG. 3) during etching. stencil masks in which through-holes 6a are etched in the thin membrane 6 (or, as shown in FIG. 2, where a patterned material 3 formed on the membrane 4 is etched), the membrane is often deformed or destroyed due to the pressure of the coolant gas impinging on it.
Additionally, conventional temperature-maintenance methods are often ineffective because the coolant gas flows through the through-holes 6a as the holes are being etched. As a result, the gas does not act to sufficiently and consistently cool the mask workpiece. Accordingly, conventional methods of maintaining the temperature of a mask workpiece during manufacture do not consistently maintain the temperature of the mask workpiece below a specified temperature during performance of the etch processes necessary to form the mask.
Previous attempts to resolve the problem of damage to the membrane by the cooling gas include strengthening the membrane (i.e., providing a thicker membrane) to allow for direct cooling of the membrane. Alternatively, membranes have been strengthened by application of a resin (e.g., a resist) to the lower surface of the membrane. Such cooling methods are insufficient to maintain the mask below required temperature levels. Further, whereas thinner membranes provide better pattern resolution, thinner membranes are less resistant to thermal damage than thicker membranes. Moreover, application (and subsequent removal) of the reinforcing resin on the membrane increases the amount of processing required to manufacture a mask and increases the risk of damaging the mask membrane.
Accordingly, there is a need for methods for making charged-particle-beam microlithography masks in which the methods provide consistent cooling of the mask workpiece especially during etching and other steps as required without causing damage to the resulting mask.